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Why would you want to try. Different clutch, gear box, crank cases and engine cases
Will they mesh, yes. Tried to do it back in the 70s but as you say the cases are ever so slightly a different shape and all the oil would just run out. I can see the attraction, the 500 clutch can be heavy if not set up right, so can the 550 TBH.My 500 clutch was really really light, everything was well greased and set up perfectly, cable routed and oiled correctly, pivot on the lever greased etc. I'm also looking at an hydraulic conversion to make it even lighter but it's still in the planning stage. You need a 550 pinion gear let me know, I should have one.